The 27-year-old actress is reprising the role of Kitty Pryde - who she first played in 2006's 'X-Men: The Last Stand' - for the forthcoming 'X-Men: Days of Future Past' and she was forced to use tennis balls as props when working on scenes which relied on computer graphics, so was able to convincingly portray fear.

On 'Late Night with Seth Meyers', she was asked: ''Where there are giant sentinels - what are you looking at [when filming]?''

Ellen replied: ''It's a tennis ball. But I have a phobia of tennis balls, so it works really well.''

Ellen admitted she has always been afraid of the small balls and can't even watch tennis on TV as the sports equipment terrifies her so much.

She added: ''I've always had it.Tennis comes on I have to get out of the room.''

The brunette beauty - who was catapulted to the spotlight for her role in 2005 drama 'Hard Candy' - later posted an image of herself on Twitter when she was backstage and can be seen clutching a coffee mug which covered much of her face.